docs: def _get_pga_on_rock(self, rup, dists, _C):
          """
          Compute and return PGA on rock conditions (that is vs30 = 760.0 m/s).
          This is needed to compute non-linear site amplification term
          """
          # Median PGA in g for Vref = 760.0, without site amplification,
          # that is equation (1) pag 106, without the third and fourth terms
          # Mref and Rref values are given in the caption to table 6, pag 119
          # Note that in the original paper, the caption reads:
          # "Distance-scaling coefficients (Mref=4.5 and Rref=1.0 km for all
          # periods, except Rref=5.0 km for pga4nl)". However this is a mistake
          # as reported in http://www.daveboore.com/pubs_online.php:
          # ERRATUM: 27 August 2008. Tom Blake pointed out that the caption to
          # Table 6 should read "Distance-scaling coefficients (Mref=4.5 and
          # Rref=1.0 km for all periods)".
          C_pga = self.COEFFS[PGA()]
          pga4nl = np.exp(self._compute_magnitude_scaling(rup, C_pga) +
                          self._compute_distance_scaling(rup, dists, C_pga))
  
          return pga4nl